A cellulose-based biomass is an effective starting material for a useful alcohol, such as ethanol, or an organic acid. In order to increase the amount of ethanol produced with the use of a cellulose-based biomass, yeast strains capable of utilizing an xylose, which is a pentose, as a substrate have been developed. For example, JP Patent Publication (Kokai) No. 2009-195220 A discloses a recombinant yeast strain resulting from incorporation of the xylose reductase gene and the xylitol dehydrogenase gene derived from Pichia stipitis and the xylulokinase gene derived from S. cerevisiae into its chromosome.
FEMS Yeast Research, vol. 9, 2009, 358-364 and Enzyme and Microbial Technology 33, 2003, 786-792 describe that culture of yeast strains provided with the xylose-metabolizing ability in a fermentation medium containing acetic acid results in lowered rates of xylose fermentation. In addition, Appl. Microbiol. Biotechnol., 2009, 84: 37-53 describes a yeast strain with the xylose-fermenting ability and a variant with an improved rate of xylose fermentation.
However, no techniques for improving the xylose-metabolizing ability of a yeast strain having such ability have been known.